Measure And Troubleshoot Digitally Modulated Signals

Digitally modulated signals are ubiquitous in modern wireless-communications systems, enabling multimedia services over limited bandwidths.The number of tests required to evaluate digitally modulated signals can be daunting.Fortunately,the Agilent MXA signal analyzer provides the many test capabilities needed to evaluate the modulation quality of modern signals.By using the MXA signal analyzer with a simple three-step procedure,it is possible to speed and simplify the measurement of troubleshooting of even the most complex signals, such as multicarrier orthogonal-frequency-division-multiplex (OFDM) signals.

The Agilent MXA signal analyzer (Fig. 1) combines the capabilities of a traditional swept-tuned spectrum analyzer and a vector signal analyzer. The midrange family of signal analyzers currently offers four versions spanning 20 Hz to 3.6 GHz, 8.4 GHz, 13.6 GHz, and 26.5 GHz. Its fast tuning speed and standards-based measurement applications might land it on the production line, but it is also a powerful research and design tool for general purpose, aerospace, and defense applications.

The MXA signal analyzers are available with a wide range of standard-specific software applications, pre-programmed according to the requirements of specific wireless-communications standards, including IEEE 802.16e WiMAX, WCDMA, HSDPA/HSUPA, and automated phase-noise measurements. The MXA analyzers can perform signal-modulation analysis on everything from digital-video (DV) and cellular systems through WiMAX wireless local-area network (WLAN), and private-mobile-radio (PMR) systems and their components, with test applications for all leading wireless-communications standards, such as Bluetooth, GSM, EDGE, TETRA 1 and 2, TD-SCDMA, and ZigBee.

The MXA signal analyzer provides extensive spectrum-analysis functions and complete vector signal analysis in a single instrument, with the capability of switching between modes in a fraction of a second. Modulation analysis on wireless signals can be challenging, often prompting test-equipment operators to commence with vector signal analysis. But a measurement sequence that begins with spectrum analysis may yield a finished design and more meaningful test results faster than trying to immediately make demodulation measurements on complex modulated waveforms. The MXA signal analyzer can be used as part of a three-step process (Fig. 2) proven effective for measuring and troubleshooting digitally modulated signals:

Begin with basic spectrum measurements.

Continue with vector measurements that combine frequency and time measurements.

This process is well suited to the design phase of research & development (R&D), although it can also be applied to design verification and other tasks typically found in prototype and pilot-scale manufacturing.This sequence improves the chances of finding signal problems at the earliest stages of a design.

Traditional spectrum measurements are fundamental and familiar, and can provide information about essential signal traits such as power, distortion, noise or signal to noise, and phase noise. Traditional spectrum measurements are also used to verify frequency-conversion operations for proper frequency and amplitude (Fig. 3). The MXA signal analyzers employ all digital resolution bandwidth filters as part of their digital intermediate-frequency (IF) section, for sharp and accurate signal differentiation during spectrum analysis.

Most digitally modulated signals vary with time, including on/off RF bursts and changes in signal composition due to equalizer training and synchronization sequences. Time-specific measurements can provide a great deal of insight into these signals. Fortunately, the MXA signal analyzer blends three tools that can simplify the analysis of time-related signal characteristics: a traditional spectrum analyzer, a VSA with the capabilities of Agilent's powerful 89601A analyzer built into an application program, and a host of standard-specific measurements that provide pre-programmed test setups for quickly getting information about a signal. Using Fast Fourier Transform (FFT) and digital-signal-processing (DSP) operations, the MXA's 89601A VSA application offers important triggering capabilities for capturing time-specific signal characteristics. These capabilities include pulse-triggering measurements with adjustable levels, holdoff, and positive/negative trigger delays to precisely select the start of the measurement interval; flexible time gates to select the desired portion of the signal for the measurement; adjustable frequency resolution and resolution-bandwidth filter shapes to optimize amplitude accuracy and time versus frequency resolution; powerful gating for all measurements, including spectrum, power occupied bandwidth, complementary cumulative distribution function (CCDF), power spectral density; time capture and replay functions for single-shot measurements-(as many as 42 million measurement points); and adjustable FFT time record size with records as large as 409,600 points, enabling single-trace measurements of even the longest and widest RF bursts.

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Using Basic Demodulation
What is the goal of basic digital demodulation? It includes the analysis and validation of essential modulation parameters, such as error vector magnitude (EVM), relative constellation error (RCE), and in-phase/ quadrature (I/Q) error parameters. It may also be a matter of correctly detecting symbol states or determining if a receiver symbol clock is locked.

Basic digital demodulation measurements require that the analyzer's frequency and time parameters are properly set. If not, demodulation will likely fail. Basic digital demodulation produces a number of results useful for evaluating modulation quality, such as EVM, RCE, and modulation error ratio (MER), along with direct in-phase/quadrature (I/Q) error measurements such as I/Q offset, quadrature error, and amplitude/gain imbalance. Frequency and amplitude stability can be evaluated by examining individual components of the vector error, such as amplitude error and phase error, along with general frequency-error parameters derived during demodulation.

The MXA's standard-specific measurement applications are a practical starting point for basic digital demodulation. Since the measurement parameters are already adjusted according to the requirements of the standard, this approach can speed and simplify measurement setup and reduce setup errors. Each application is preprogrammed with the most useful and most common tabular (Fig. 4) and graphical displays, and provides customized features such as preset limit lines and pass/fail testing.

Graphical error-vector measurements are particularly useful for troubleshooting. A good starting point is analysis of errors as a function of time. Using the MXA signal analyzer's various demodulation applications, the instrument generates a perfectly modulated reference signal for comparison to the received symbol sequence. The analyzer performs vector subtraction of the reference from the signal of interest for each symbol instant and for times in between. It then produces a time-based vector quantity that can be displayed according to seconds or symbol times. These vector error measurements show residual signal quantities after the ideal modulation has been removed from the actual modulation.

Analysis of vector errors as a function of time can reveal problems associated with turn-on/turn-off events, impulsive interference, operation of automatic-gain-control (AGC) circuits, and fast fading phenomena. This is useful for signals such as WiMAX (both fixed and mobile) where modulation schemes are changed during RF bursts. Because these errors are vector quantities, they can be transformed (via FFT) into the frequency domain for error vector spectrum analysis.

Marker coupling (Fig. 5) is an effective means of evaluating different error measurements. Part of the MXA's 89601A VSA application, it allows an operator to link markers on as many as four to six simultaneous measurement traces for analysis of error peaks in different domains ( frequency, time, and I/Q). For example, an error peak found in a vector time trace can be checked with an I/Q constellation or vector display to determine if the peak is associated with a symbol transition or an amplitude anomaly, such as compression.

Adding Advanced Demodulation
The last step in the measurement and troubleshooting sequence employs advanced demodulation for finding complicated problems, such as those associated with WiMAX and OFDMA. Advanced demodulation techniques include demodulation of specific portions of signals in time and frequency and/or use of more advanced demodulation operations such as adaptive equalization and configurable pilot tracking (primarily for OFDM). In the MXA signal analyzer, these advanced demodulation techniques are available in the MXA's 89601A VSA measurement application, whether running in the instrument itself or on an external computer connected via the MXA signal analyzer's USB, LAN, or GPIB port.

Some advanced modulation techniques take advantage of common signal-practices, like adaptive equalization, typically used to overcome the effects of multipath and non-flat fading in communications systems. In systems employing adaptive equalization, it is useful to understand received signal quality both before and after equalization. It may be preferable to evaluate a full receiver with equalization applied, while it may be more meaningful to test system components, such as frequency converters, filters, and modulators, without equalization in order to optimize their "raw" performance. The MXA signal analyzer's 89601A VSA measurement application includes both a general adaptive equalization function (for most single-carrier modulation types) and standard-specific equation for particular systems, such as WiMAX.

The "triple-threat" measurement capability of the MXA signal analyzer with the 89601A VSA measurement application provides the troubleshooter with high-speed spectrum analysis, advanced vector signal analysis, and standard-based measurement capabilities that can help speed and simplify evaluation of digitally modulated signals in wireless-communications systems.

Note: This White Paper is based on Application Note 1585, which is available for free download from Agilent Technologies at www.agilent.com/find/MXA_AN1585.